Sahih Bukhari Volume 2, Book 20, Hadith Number 213.

Narated By Anas bin Malik : Whenever the Prophet started the journey before noon, he used to delay the Zuhr prayer till the time for the ‘Asr prayer and then he would dismount and pray them together; and whenever the sun declined before he started the journey he used to offer the Zuhr prayer and then ride (for the journey).